Add Dominoes exercise for Elixir track

elixir/dominoes/lib/dominoes.ex

@@ -0,0 +1,22 @@
+defmodule Dominoes do
+  @type domino :: {1..6, 1..6}
+
+  @doc """
+  chain?/1 takes a list of domino stones and returns boolean indicating if it's
+  possible to make a full chain
+  """
+  @spec chain?(dominoes :: [domino]) :: boolean
+  def chain?([]), do: true
+  def chain?([{first, second}]) when first == second, do: true
+
+  def chain?([{first, second} | dominoes]) do
+    Enum.any?(
+      dominoes,
+      fn
+        {^first, x} = domino -> chain?([{second, x} | List.delete(dominoes, domino)])
+        {x, ^first} = domino -> chain?([{second, x} | List.delete(dominoes, domino)])
+        _ -> false
+      end
+    )
+  end
+end